Shelly is a very strong trainer. In Sapphire and Emerald she appears first at the weather institute and then at the Seafloor Cavern. In your first battle against her, she has a Carvanha and a Mightyena. At the Seafloor Cavern the only change is that her Carvanha has evolved into Sharpedo. Shelly doesn't appear in Pokémon Ruby.

In the anime Shelly has a Carvanha but instead of a Mightyena she has a Walrein and a Crawdaunt, but she is a scuba diver.